In the development of radio communication systems, such as mobile communication systems, efforts are made for an evolution of the radio access part thereof. In this regard, the evolution of radio access networks (like for example the GSM EDGE radio access network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN) or the like) is currently addressed. Such improved radio access networks are sometimes denoted as evolved radio access networks (like for example the Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) or as being part of a long-term evolution (LTE) or LTE-Advanced. Although such denominations primarily stem from 3GPP (Third Generation Partnership Project) terminology, the usage thereof hereinafter does not limit the respective description to 3GPP technology, but generally refers to any kind of radio access evolution irrespective of the underlying system architecture. Another example for an applicable broadband access system may for example be IEEE 802.16 also known as WiMAX (Worldwide Interoperability for Microwave Access).
In the following, for the sake of intelligibility, LTE (Long-Term Evolution according to 3GPP terminology) or LTE-Advanced is taken as a non-limiting example for a broadband radio access network being applicable in the context of the present invention and its embodiments. However, it is to be noted that any kind of radio access network may likewise be applicable, as long as it exhibits comparable features and characteristics as described hereinafter.
In the development of cellular systems in general, and access networks in particular, relaying has been proposed as one concept. In relaying, a terminal or user equipment (UE) is not directly connected with an access node such as a radio base station (e.g. denoted as eNodeB or eNB) of a radio access network (RAN), but via a relay node (RN) which is connected to the access node. Relaying by way of relay nodes has been proposed as a concept for coverage extension in cellular systems. Apart from this main goal of coverage extension, introducing relay concepts can also help in providing high-bit-rate coverage in high shadowing environments, reducing the average radio-transmission power at the a user equipment (thereby leading to long battery life), enhancing cell capacity and effective throughput, (e.g. increasing cell-edge capacity and balancing cell load), and enhancing overall performance and deployment cost of radio access networks.
FIG. 1 shows a schematic diagram of a typical deployment scenario of a relay-enhanced cellular system, such as e.g. a LTE or LTE-Advanced RAN with radio-relayed extensions, for which exemplary embodiments of the present invention are applicable. As shown in FIG. 1, UEs, e.g. those located at disadvantaged positions such as a cell edge and/or high shadowing areas, are connected to a so-called donor base station (DeNB) via a respective relay node RN. Generally, any one of the relay nodes may be stationary/fixed or mobile.
The coverage or service area of a relay node may be referred to as relay cell, and the coverage or service area of a donor base station may be referred to as donor cell. Accordingly, both the DeNB as well as the RNs may be regarded as access nodes or base stations of an access network, possibly as access nodes or base stations of different hierarchical levels in terms of logical and/or structural network deployment.
In a relay-enhanced cellular system, a relay node acts as a user equipment (UE) from the point of view of its serving donor base station (DeNB) and as a base station (eNB) from the point of view of its served user equipment or terminal (UE) of an actual user. Accordingly, a relay node, also referred to as relay base station or relay cell hereinafter, supports both UE and eNB functionality and, thus, incorporates both UE and eNB functions.
FIG. 2 shows a schematic diagram of a system architecture of a relay-enhanced cellular system, such as e.g. a LTE or LTE-Advanced RAN with radio-relayed extensions, for which exemplary embodiments of the present invention are applicable. As shown in FIG. 2, further network entities and/or functions are involved, such as a mobility management entity/function (MME) for the RN-UE function and the user terminal, a serving gateway (SGW) and a packet data network gateway (PGW) entity/function for the RN-UE function and the user terminal, as well as an optional relay gateway (GW) entity/function. While various alternative implementations are conceived (being indicated the blocks denoted as Alt. 1, Alt. 2 and Alt. 3), the implementation according to Alt. 2 is currently specified as standard.
The individual entities/functions are linked by specified interfaces indicated between respective blocks in FIG. 2. In particular, the (wireless) link between donor base station (DeNB) and relay node (RN) is referred to as Un link or relay link, and the (wireless) link between the relay node (RN) and the terminal or user equipment (UE) is referred to as Uu link or access link.
The following specification particularly relates to the case of mobile relay nodes as well as specific issues and problems thereof in comparison with stationary/fixed relay nodes.
An important use case in relaying is group mobility, where the relay node is moved when serving its UE or UEs. For example, such mobile relay cell scenario applies when the relay node is installed in a moving entity, such as a (high speed) train, and serves the UEs of the people traveling e.g. in the (high speed) train. In such mobile relay cell scenario, the relay node will have to connect to various donor base stations during the traveling e.g. of the (high speed) train, which are installed along the rails. Such mobile relay cell scenario is exemplarily illustrated in FIG. 3.
According to the current specifications, a relay node appears to its UE or UEs as a distinct cell with respect to its donor cell. Therefore, each relay node cell should be configured with a different physical cell identifier (PCI). To avoid confusion and/or collision at the UE side, it is required that each cell (including each relay node cell) is configured with a different PCI among its surrounding neighboring cells. To solve the PCI confusion and/or collision issue, a PCI selection framework is currently specified, wherein one or more PCIs are selected for each D/eNB by a centralized or distributed algorithm before operating. Thereby, the PCI confusion and/or collision regarding the D/eNB could usually be avoided, since the D/eNB maintains its PCI during operation. That is to say, as the D/eNB does usually not move, it is not necessary to change its PCI during operation, since no the PCI confusion and/or collision could arise e.g. due to a changing network environment.
However, for mobile (moving) relay nodes, the situation is substantially different.
The selected PCI for the relay node cell is uniquely defined by the algorithm when operating at one place. While, during the period of moving, some new neighbors may appear, the previously selected PCI of a mobile (moving) relay node cell may not be unique anymore at the new place. To avoid the PCI confusion and/or collision, it is thus required to change the old PCI to a new PCI which is unique at the new place. Given that many UEs could be served by the relay node cell during the moving period, the change of the PCI on the fly raises a new issue for mobile (moving) relay nodes. Namely, the change of the PCI would typically interrupt the service continuation of UE/UEs currently served by the relay node due to a radio link failure (RLF) detected by the UE/UEs.
Accordingly, when the on-the-fly PCI change at the relay node is required, the negative impact to UE/UEs currently served by the relay node is to be avoided or at least reduced.
FIG. 4 shows a time chart illustrating an exemplary RLF detection and RRC connection re-establishment process.
As illustrated in FIG. 4, in case the PCI is changed (thus terminating the preceding normal operation), the served UE/UEs will no longer detect the synchronization signal indicated by the old PCI of the serving cell and enter a RLF recovery and RRC connection re-establishment process.
Namely, when the value of a maximum number of out-of-sync indications (N310) is exceeded, a RLF timer (T310) is started so as to start a RLF discovery (sync recovery) process. When the sync recovery fails until lapse of the RFL timer (T310), a RLF is detected, and a RRC connection re-establishment process is initiated.
According to current specifications, the radio link quality estimation period for out-of-sync indications could be set to 200 ms. Usually, the default values for N310 and T310 are N310=1 and T310=1000 ms, respectively. Thus, the typical time from PCI change (i.e. from the time when a UE can not receive the cell-specific reference signal) to RLF detection (i.e. the time when UE detects RLF) is about 1200 ms. This long interruption time (delay) would seriously impact the experience of users. Therefore, mechanisms to improve user experience during an on-the-fly PCI change at a base station or cell, such as for example a mobile (moving) relay node or relay node cell, are desirable.
As stated above, the change of PCI on the fly is not required for D/eNBs or fixed relay nodes, since the existing PCI selection approach is effective for such base stations. Accordingly, there is currently not specified any solution to the above-outlined issues and problems of mobile (moving) relay nodes.
According to certain considerations in this regard, it could be conceivable to avoid or resolve PCI confusion/collision in case of mobile (moving) relay nodes. For example, if collision avoidance fails and, during collision resolution, the moving relay node is selected to change the PCI, it could be conceivable that the UEs served by the moving relay node are handed over to the DeNB or to other nodes (base stations). However, this is not always possible, e.g. in case the neighboring cells are overloaded, and the UEs connected to the RN would thus experience a radio link failure and service interruption. The UEs served by the relay node could typically be moving with the moving relay node, and the signal from neighboring nodes might be strongly attenuated by the vehicle upon which the relay node is installed, which is why handing over these UEs to the DeNB or other nodes might spoil the performance in the neighboring cells. Otherwise, it could also be conceivable to reserve a part of available PCIs for mobile relay nodes, and to keep the PCI of moving relay nodes unchanged. Thereby, PCI confusion and collision between relay nodes and other neighboring cells could be avoided, but there would still exist a problem of a potential PCI confusion and/or collision between different relay nodes.
In view thereof, there do not exist any mechanisms for enabling cell reconfiguration (for example, in a relay-enhanced network environment) in a proper an efficient manner. Accordingly, such mechanisms are needed.